The present invention relates to an operation method for a secondary battery that is repeatedly charged and discharged for use and a secondary battery device.
For example, a secondary battery device mounted on an electric vehicle provides energy for the vehicle to travel by charging a secondary battery with a charger to a predetermined charge end voltage and then discharging the battery as required. Besides, in order to keep a temperature of the secondary battery within a certain range in an environment of low or high temperature, a temperature controller having a cooling fan or heater is typically provided in the secondary battery device.
Charge control by the charger in such a secondary battery device is carried out in the following manner. That is, in the case of a lithium secondary battery, for example, constant current charge is first conducted with a constant current, constant voltage charge is then conducted for a predetermined time with a charge end voltage of, for example, 4.2 V, and thereby the secondary battery is charged to 100% of a quantity of charged electricity.
By the way, for leveling the electric power demand, some office buildings are equipped with a secondary battery device for storing a midnight electric power, for example. This secondary battery device comprises a secondary battery, a charger for charging the secondary battery with the midnight electric power, and an inverter for generating an alternating current power in the daytime from a DC power stored in the secondary battery.
Charge control by the charger in such a secondary battery device for load-leveling is carried out in the following manner. That is, in the case of a lithium secondary battery, for example, when the time period of the midnight electric power comes round, constant current charge is first conducted with a constant current, and upon attaining a predetermined voltage, the charge scheme is shifted to constant voltage charge, which is conducted for a predetermined time.
Such a secondary battery has a disadvantage that raising the charge end voltage to enhance a charging rate promotes degradation of the battery, resulting in a reduced life thereof. This problem has been pointed out in Japanese Patent Laid-Open No. 11-4549, in which extension of the battery life is attempted by stopping charge before the secondary battery is fully charged to suppress the quantity of charged electricity.
While the battery life can be extended by suppressing the quantity of charged electricity as described in the above Japanese Patent Laid-Open No. 11-4549, however, the reduced quantity of charged electricity poses another problem of a reduced utilization factor of the secondary battery. Thus, the secondary battery cannot be fully used; for example, a driving range of the electric vehicle may be reduced. Although such a circumstance is preferable for a user placing prime importance on the battery life, it is not preferable for a user placing prime importance on the capacity rather than the life thereof. Furthermore, in the case of the secondary battery device for load-leveling, since the required capacity of the secondary battery is predetermined in the design stage depending on a maximum power consumption by load, if the utilization factor of the secondary battery is reduced, the number of secondary batteries provided has to be increased accordingly, and therefore, the cost of equipment will be increased.
The present invention is devised in view of the above described circumstances, and a first object of this invention is to provide an operation method for a secondary battery and a secondary battery device that allow a user to decide which is to be given a priority, the life of the secondary battery or capacity thereof. In addition, a second object of this invention is to provide an operation method for a secondary battery and a secondary battery device that can provide an extended life of the secondary battery and an enhanced utilization factor thereof.
According to a first aspect of this invention, in a charge operation of a secondary battery, at least two operation modes including first and second operation modes in which the secondary battery is charged to predetermined charge end voltages different from each other are established, and either of the two operation modes can be selected according to a control signal from the outside of the secondary battery. In an operation mode with high charge end voltage, high battery capacity is provided, so that it is suitable for a case where a user wishes to travel a long distance by an electric vehicle, for example. On the other hand, in an operation mode with low charge end voltage, although the battery capacity is reduced, the life of the battery is extended, so that the running cost thereof is advantageously reduced. The user can select either of the modes arbitrarily.
In the case of a secondary battery device for load-leveling installed in an office building, for example, while power consumption by load is high on weekdays due to the operation of office automation appliances and an air conditioning system in the building, the power consumption by load is low on a holiday due to the significantly reduced rate of operation thereof. In this way, in a significant number of cases, the power consumption by load of the secondary battery device varies in a predetermined pattern. In such a secondary battery device, there should be no problem if the quantity of charged electricity of the secondary battery is suppressed before the load is reduced.
Thus, according to a second aspect of this invention, a timer device switches the operation mode between the first operation mode and the second operation mode that are different in charge end voltage. By the timer device establishing the first operation mode with high charge end voltage when the power consumption by load is high and the second operation mode with low charge end voltage when the power consumption by load is low, degradation of the secondary battery can be minimized while assuring a required quantity of electric power.
Alternatively, the first and second operation modes may be switched to each other according to the battery temperature. When the battery temperature is higher than a predetermined value, the battery is charged in the second operation mode with low charge end voltage. This is because when the battery temperature is high, the impedance of the battery is reduced and the quantity of charged electricity tends to increase, and the battery tends to be readily degraded, and therefore, it is desired that the life of the battery is extended by lowering the charge end voltage. Here, the battery temperature may be measured with a temperature sensor attached to a battery case, or a temperature of the air surrounding the installed battery may be measured and regarded as the battery temperature.
Alternatively, the first and second operation modes may be switched to each other according to a state of health (SOH) of the battery. When the state of health of the battery is equal to or higher than a predetermined value, the battery is charged in the second operation mode with low charge end voltage, and when the state of health is equal to or lower than the predetermined value, the battery is charged in the first operation mode with high charge end voltage. When the state of health of the battery is high, the capacity of the battery is high and the internal resistance thereof is low, so that operation with low charge end voltage is possible, and thus, degradation of the battery is hard to advance. When the state of health of the battery is reduced, increasing the charge end voltage enables an extended period of use of the battery. Here, the state of health of the battery can be determined based on a voltage drop value and the battery temperature during discharge of the battery.
Alternatively, in the case of a sealed battery, since as degradation of the battery advances, the pressure in the battery is increased, the internal pressure of the battery may be measured, and the modes of the battery may be switched to each other according to the internal pressure. In this case, if degradation of the battery does not advance, and the internal pressure is low, degradation of the battery can be suppressed by operation with low charge end voltage. When the internal pressure is increased due to degradation of the battery, increasing the charge end voltage enables an extended period of use of the battery. Here, the internal pressure of the battery can be measured with a pressure sensor arranged in the battery, or a strain gage attached to a surface of the battery case.
In the case where a battery temperature controlling device for controlling the battery temperature and the sensor for detecting the state of the battery are provided, and the battery is charged in the first operation mode with high charge end voltage, it is desired that the temperature of the secondary battery during charge and discharge is higher than that in the second operation mode. This is because the higher temperature allows the stored energy to be released more efficiently and a larger service capacity to be assured. When a state of charge of the battery is high, it is desired that the operating temperature of the battery is low. This is because while the battery tends to readily be degraded when the state of charge of the battery is high, lowering the operating temperature enables the degradation to be suppressed. In addition, when a discharge current of the battery equal to or lower than a predetermined value is detected, it is desired that the operating temperature of the battery is low. This is because when the load current is low, a low operating temperature is sufficient for operation, as well as can lead to suppression of the degradation of the battery.